The present invention relates to a system capable of retrieving information from an optical recording medium memory, and more specifically, to an information retrieving system in which light beams from two or more light sources are converged on an optical memory.
Conventionally, there are various types of information retrieving systems in which light beams from two or more light using sources are converged on an optical memory. The optical memory used in such systems can be a memory used exclusively for retrieving data, a memory for recording and retrieving data or a memory capable of recording, retrieving and erasing data. They can be in the form of a disk, card or other appropriate medium. Designed using the appropriate memory, the information retrieving system can be a read-after-write type in which information is retrieved immediately after it is recorded on the optical memory by means of a recording laser beam. Another example is a write- or read-after-erase type in which new information is recorded on the optical memory immediately after previously recorded information is erased by means of an erasing laser beam, or in which unerasable pre-information pits are retrieved while information recorded on the optical memory is being erased. The information retrieving systems of these types include a focusing servo system for accurately controlling the focus of the recording or retrieving laser beam on the optical memory, and a tracking servo system for tracking accurately with the recording or retrieving laser beam. In an optical system using the conventional focusing servo system, if two laser beams impinge on one photodetector for focus detection, the focusing state of an objective lens cannot be detected accurately. Therefore, different wavelengths are used for the two laser beams. Also, an optical separator, such as a dichroic mirror, is disposed in the optical path of the laser beams returned from the optical memory. Thus, each laser beam is divided into two beams, only one of which is introduced into the photodetector for focus detection. In the optical system of this type, the optical separator must be located in the optical path, so that the optical path must be relatively long. Accordingly, the optical system tends to be large and heavy in weight. Usually, moreover, the optical system is incorporated in an optical head which is moved by a linear actuator. If the optical head is heavy in weight, it takes an excessive amount of time to move the head. Thus, it requires much time to access recorded information.
These problems can be settled by optical systems which are disclosed in U.S. patent application Ser. Nos. 673,764; 705,305; and 713,598, filed on Nov. 21, 1984; Feb. 25, 1984; and Mar. 19, 1985, respectively, by the present applicant Ando. In these optical systems, however, a light shielding plate must be located accurately in the optical path, so that the adjustment and assembly of the system are difficult.